1. Field of the Invention
The present invention relates to a fixing mechanism of a screw of an injection molding machine.
2. Description of Related Art
In general, an injection molding machine includes, as a pedestal, a screw driving portion connected to a power source such as a motor to rotate a screw and to move the screw forward and backward, wherein the screw is mounted to the screw driving portion. There are various types of structures for mounting the screw. One of such mounting structures is a structure for connecting the screw to the screw driving portion by forming a notch in the screw and inserting members called retainers into the notch. For example, the above-described notch in the screw is pinched in a set of two retainers in semicircular shapes obtained by splitting a ring and a fixing bushing is fitted and fixed to cover the retainers. Keys or splines to be fitted with each other are formed in advance on the screw and the fixing bushing and forces for moving the screw forward and backward and rotating the screw are transmitted to the screw through the retainers and the fixing bushing. This screw mounting structure in which the semicircular retainers is used is often employed in the molding machine.
On the other hand, the screw needs to be detached frequently from the screw driving portion for cleaning, change of the screw due to change of a mold, and the like. In this case, in the above-described connection using the semicircular retainers, the fixing bushing needs to be detached first from the screw driving portion to touch the retainers. However, the fixing bushing is fixed to the screw driving portion by using many bolts in order to transmit large power to the screw and therefore it takes much work to detach the fixing bushing. If the screw is detached frequently, poor workability in changing the screw results in poor productivity.
Therefore, in order to improve workability, some structures, with which the screw can be easily detached easily by detaching only the retainers without detaching the fixing bushing, have been proposed.
There is a structure as disclosed in Japanese Utility Model Application Laid-Open No. 7-42689, in which a hole into which a screw is inserted and a hole orthogonal to an axial direction of the screw are formed in a screw mounting portion and a retainer is inserted into the hole orthogonal to the axial direction of the screw so as to pass through notches in the screw.
There is a structure as disclosed in Japanese Patent Application Laid-Open No. 8-118438, in which splines and notches are formed in a screw, a fixing bushing having grooves is connected through the splines to the screw, and retainers fitted in the notches through the grooves protrude outside from the fixing bushing.
However, these days, output of a motor or the like is increasing and it is sufficiently possible to drive the screw driving portion even if an output of the motor is directly connected to the screw driving portion without speed reduction by a pulley or the like. In such a structure, the screw driving portion tends to be designed to be small and light to reduce an inertial force so as to improve performance in operation.
In the structure disclosed in Japanese Utility Model Application Laid-Open No. 7-42689 described above, the retainer fixes both rotation and forward and backward movements of the screw. When torque to be transmitted and power for the forward and backward movements are large, the screw is restricted in terms of strength. Moreover, because the hole into which the screw is inserted is formed in the connecting portion, it is difficult to adapt to a wide range of screw diameters.
In the structure disclosed in Japanese Patent Application Laid-Open No. 8-118438 described above, retainer fixing portions are provided to the screw driving portion on an outer peripheral side of the fixing bushing. Therefore, if the screw driving portion is as small as the fixing bushing, it is impossible to form bolt holes and pin holes for fixing the retainers and, as a result, the retainers cannot be fixed.
Moreover, because the retainers are mounted to the screw driving portion, clearances are formed between the fixing bushing and front sides of the retainers. Therefore, the retainers are supported by retainer fixing bolts when the screw moves backward and forces in bending directions are likely to be applied to the retainers and the retainer fixing bolts.
Furthermore, if the retainer fixing bolts are detached, the retainers come off, which may causes the retainers to drop off during work. Moreover, because the retainers have such widths as not to interfere with fixing bolts of the fixing bushing, the retainers cannot have large widths and are restricted in terms of shape if many fixing bolts are used.